windows: use proper system directory path in fallback loader

The %WINDIR% variable is an odd choice and not even entirely reliable.
Since Windows 2000, there has been a specific function for determining
this information, so let's use it. It's also a useful function in its
own right for folks who want to launch system tools in a somewhat safe
way, like netsh.exe.

Updates golang/go#14959
Updates golang/go#30642

Change-Id: Ic24baf37d14f2daced0c1db2771b5a673d2c8852
Reviewed-on: https://go-review.googlesource.com/c/sys/+/165759
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/syscall_test.go

@@ -7,6 +7,7 @@
 package windows_test
 
 import (
+	"strings"
 	"syscall"
 	"testing"
 
@@ -51,3 +52,13 @@ func TestGetProcAddressByOrdinal(t *testing.T) {
 		t.Error("shlwapi.dll:IsOS(OS_NT) returned 0, expected non-zero value")
 	}
 }
+
+func TestGetSystemDirectory(t *testing.T) {
+	d, err := windows.GetSystemDirectory()
+	if err != nil {
+		t.Fatalf("Failed to get system directory: %s", err)
+	}
+	if !strings.HasSuffix(strings.ToLower(d), "\\system32") {
+		t.Fatalf("System directory does not end in system32: %s", d)
+	}
+}